I downloaded the first savegame (the very beginning) and I started playing independently... I know it's not really the spirit of a succession game but I didn't really want to intrude.
So, at the beginning I did exactly as you guys.
Do not settle, attack and conquer istambul to make it a cool capital.

Also for me the former roman legion survives against all odds: those guys are really tough.
Some towns flip but Babylon does not declare war against me: this is the main a crucial difference between our games.
I also have to fend several counter attacks... but that's good, it's easier to destoy greeks units staying on the defence.
Feeling safe on the southern border I concentrated all my troops against Greece, every town dedicated to the war effort.
I really needed all possible units because even with a pandemic hitting all the world except me, the Greeks were hard to fix.
They have well experienced and upgraded units that unlike my units don't die by the plague.

Thats' a very bad setbak.
The human cost is umbearable... but I get Athen, and after a while also Solona.
Only Pompei (yes in Italy) remain in Greek hands.
Once won against Greece I need to build better infrastructures, improve economy and do some tech.
Euro powers are very strong and exchange tech with each other... in short time Turkey has few friends and nothing to trade.

This is a huge problem, usually with Turkey I do try to submit others into vassallage.
But in this game I fell too behind in tech to do so.

I got a decent army, lot of xp thanks to vassallage and thocracy, but it will be obsolete in very short time.
How and where I can get a vassal?
The Netherlands offer themselves so I could protect them.
I gladly accept (good luck).
But still it's 1470 and I don't have any town in mesopotamia!
I did bring my fully restored, upgraded, experienced and increased army near the border, one step in and I can bomb their capital.
An other town is up-right, and shiraz not too far.
result: 1495 Mesopotamia is conquered and babylon annihilated.
I do have the territory gains to complete the 1st 2 steps of UHV, and 300 years to get other 2 vassal.
But whom?
France and germany are too strong: bigger and better army, superior tech.
Spain is vassal of France, Egypt is vassal of Germany.
Maybe Arabia is a possible target... hopefully they'll not fight to the bitter end but give capitulation instead.